Semiconductor wafer fabrication can use plasma processing to manufacture semiconductor devices, such as microprocessors, memory chips, and other integrated circuits and devices. Plasma processing involves energizing a gas mixture by introducing RF energy. This gas mixture is typically contained in a vacuum chamber, also called plasma chamber, and the RF energy is introduced through electrodes or other means in the chamber. In a typical plasma process, the RF generator generates power at an RF frequency and this power is transmitted through RF cables to the plasma chamber.
To provide efficient transfer of power from the generator to the plasma chamber, a matching network is used. The purpose of the matching network is to transform the plasma impedance (usually a low value) to a value suitable for the RF generator. In many cases, particularly in wafer fabrication processes, the RF power is transmitted through 50 Ohm coaxial cables and the system impedance of the RF generators is also 50 Ohm. On the other hand, the impedance of the plasma, driven by the RF power, varies and this impedance must be transformed to non-reactive 50 Ohm (i.e., 50+j0) for maximum power transmission. RF matching networks perform this task of continuously transforming the plasma impedance to 50 Ohm for the RF generator. An RF matching network can include one or more variable capacitors and a microprocessor-based control circuit to control the capacitance values of the variable capacitors. The value and size of the variable capacitors are influenced by the power handling capability, the frequency of operation, and the impedance range of the plasma chamber.
The predominant variable capacitor in use in RF matching networks is the Vacuum Variable Capacitor (VVC). Electronically Variable Capacitor (EVC) technology, however, is emerging as an alternative, as EVCs can be switched more quickly. Faster switching enables faster matching, which enables faster manufacturing.
EVCs comprise discrete capacitors that are switched in or out to alter the total EVC capacitance. In such RF switching arrangements at high currents and high voltages, however, switch parasitic capacitances can play a detrimental role. There is need for a switching circuit that can cancel parasitic capacitances.